The present invention relates to battery charging, and more particularly to a method and apparatus for charging a battery utilizing heterogeneous reaction kinetics.
Batteries are devices that convert chemical energy contained in active materials directly into electrical energy by means of an oxidation-reduction electrochemical reaction involving the transfer of electrons from one material to another. Batteries are characterized as primary batteries and secondary batteries. Both types of batteries are widely used. Secondary batteries are particularly popular because they can be recharged, i.e. the state of the battery is restored.
Rechargeable or secondary batteries are recharged using chargers which fall into two broad classes: simple chargers, and closed-loop or feedback chargers. Simple chargers deliver a low level charge current to the battery over a timed interval. The current level is chosen to prevent damage to the battery due to overcharging. Feedback chargers, on the other hand, monitor the state of the battery in order to control the magnitude of the charge current during the charge cycle. The charge cycle is composed of a high current phase and a regulation phase. During the high current phase, the feedback charger applies a high charge current to the battery in order to rapidly charge the battery. The feedback charger continues to monitor the state of the battery and reduces the charging current as the charge state of the battery is restored.
The capacity and cycle life expectations of batteries depend on many different factors. Charge parameters are particularly important and strongly influence the charge capacity of the battery.
In U.S. Pat. No. 5,179,335 assigned to the common owner of the subject invention, a method and apparatus for controlling the charging process through the resistance free voltage was disclosed. The resistance free voltage represents the charge acceptance voltage of the battery. The charge acceptance voltage is calculated during a diagnostic current interruption as disclosed in U.S. Pat. No. 5,179,335 or calculated from the terminal voltage determined from the charging current and the internal charge resistance when the charging current is varied as taught in pending U.S. patent application Ser. No. 08/621,930 also assigned to the common owner of the subject invention. As further disclosed in U.S. patent application Ser. No. 08/621,930, the calculated charge acceptance voltage is compared with a temperature compensated reference voltage and the charging current is adjusted so that the measured or calculated charge acceptance voltage does not exceed the set reference voltage which has been temperature compensated.
Although the process has been successfully used for many years and is superb compared to other charging methods, there are some disadvantages. The present invention addresses these disadvantages by providing a novel method and apparatus for charging batteries.
It has been found that the resistance free or charge acceptance voltage cannot be used as the only criterion for representing the charge acceptance ability of a battery, i.e. the capability of a battery to take a maximum charging current without damage. Even for the same type of battery and battery chemistry, the charge acceptance voltage is influenced by many factors such as the internal temperature, battery age and previous charging cycles. The internal temperature parameters, plate and electrolyte, are particularly important and influence the resistance free or charge acceptance voltage readings. It will be appreciated that any attempts to compensate the charge acceptance voltage are limited by the difficulty in measuring the actual internal temperature of the battery. In practical systems only the external battery temperature is available for measurement. Moreover, in a fast charge system, a high charging rate (current) can cause a significant temperature gradient within the battery, thereby magnifying the effect of temperature on the charge acceptance voltage and the control of the charging process.
In another aspect, the present invention provides a method and apparatus which utilizes a new parameter or indicator for assessing battery charge acceptance for controlling the charging process. The method and apparatus advantageously overcomes the problems associated with the dependence of charge acceptance voltage on temperature.
According to one aspect of the present invention, the battery charge acceptance ability is determined from a terminal voltage profile taken during a variation in a diagnostic current. The variation period is preferably in the range of 4 to 1800 ms in duration. A larger variation period may be used, but will tend to slow down the charging process.
According to the invention, the terminal voltage profile will have distinctive characteristics and two terminal voltage profiles are provided on the basis of the battery chemistry.
The first group of batteries having a characteristic terminal voltage profile comprise lead-acid, nickel-cadmium, etc . . . batteries. The terminal voltage profile is characterized by an increasing profile slope during the current variation. It is believed that the profile slope is due to the increasing mass transport resistance. According to this aspect of the invention, at the beginning of the charging process, i.e. when the battery is discharged and the actual charging rate is below the battery charge acceptance, the voltage profile is almost flat during the current variation. As the battery charges, the slope of the voltage profile increases until the charge acceptance ability of the battery is reached. Once the charge acceptance ability of the battery is reached, the charging current is reduced in order to match the diminished charge acceptance ability of the battery.
The second group of batteries having a characteristic terminal voltage profile comprises nickel-metal hydride batteries. The terminal voltage profile for the second group exhibits the greatest slope at the beginning of the charging process, i.e. when the battery is fully discharged. As the battery charges, the slope of the voltage profile decreases, and approaches zero when the charging rate has reached the charge acceptance ability of the battery. The mechanism of the voltage profile for these batteries may be explained by the hydrogen absorbing alloy becoming saturated as the battery is charged resulting in hydrogen transport rate decrease.
According to another aspect of the present invention, the slope of the terminal voltage profile is used in the charging process to assess the charge acceptance ability of the battery and thereby control the charging current. In one embodiment, the slope of the voltage profile is used to control the reference voltage setpoint (SVref) so that an optimum value is obtained for the particular battery pack being charged, thereby providing a charge acceptance value based on the actual battery pack. The voltage profile is determined during a current variation interval. An advantage of utilizing the voltage profile slope according to the present invention is the elimination of the need to compensate the reference setpoint voltage for the effects of temperature.
In another embodiment, the voltage profile is determined during the charging current ramp-up phase at the beginning of the charging cycle. Once the maximum charging current is reached, the voltage profile is updated periodically or after certain percentages of the terminal voltage rises are achieved to assess the charge acceptance ability of the battery. Once the charge acceptance ability is reached, the charging current is decreased in discrete steps so that the charging current does not exceed the diminishing charge acceptance ability for the battery. The step change in the charging current advantageously provides a means for determining the voltage profile and thereby the battery charge acceptance ability without further current interruptions.
In a first aspect, the present invention provides an apparatus for charging a rechargeable battery, said apparatus comprising generator means for generating a charging current having a variable level, and during a first charging period said current having a level to rapidly charge the battery at a rate in amperes greater than the capacity in ampere-hours of the battery; controller means for controlling said generator means, said controller means including (a) means for varying said charging current for a predetermined variation interval; (b) means for generating a terminal voltage profile for the battery in said variation interval; (c) means for determining a charge acceptance ability for the battery from said terminal voltage profile; and (d) means for controlling the level of said charging current in response to said charge acceptance ability.
In a second aspect, the present invention provides a method for charging a rechargeable battery comprising the steps of: (a) generating a charging current having a variable level; (b) maintaining said charging current during a first charging period at a level to rapidly charge the battery at a rate in amperes greater than the capacity in ampere-hours of the battery; (c) changing the level of said charging current for a predetermined interval; (d) generating a terminal voltage profile for the battery in said predetermined interval; (e) determining a charge acceptance ability for the battery from said terminal voltage profile; and (f) controlling the level of said charging current in response to said charge acceptance ability.
In another aspect, the present invention provides a method for determining charge acceptance ability for a battery during the charging cycle when a charging current is applied to a rechargeable battery, said method comprising the steps of: (a) ramping the charging current up to predetermined level; (b) obtaining a first slope reading by measuring the rate of change of the terminal voltage of the battery during said current ramping stage; and (c) determining the charge acceptance ability for the battery from said first slope reading.